1. Field of the Invention
The present invention relates to voltage regulator circuits, and more particularly to digital control over a switched mode voltage regulator using dual feedback loops for improved regulation.
2. Description of Related Art
Switched mode voltage regulators are known in the art to convert an available direct current (DC) level voltage to another DC level voltage. A switched mode voltage regulator provides a regulated DC output voltage to a load by selectively storing energy in an output inductor coupled to the load by switching the flow of current into the output inductor. A buck converter is one particular type of switched mode voltage regulator that includes two power switches that are typically provided by MOSFET transistors. A filter capacitor coupled in parallel with the load reduces ripple of the output current. A pulse width modulation (PWM) control circuit is used to control the gating of the power switches in an alternating manner to control the flow of current in the output inductor. The PWM control circuit uses feedback signals reflecting the output voltage and/or current level to adjust the duty cycle applied to the power switches in response to changing load conditions.
Conventional PWM control circuits are constructed using analog circuit components, such as operational amplifiers and comparators. But, it is desirable to use digital circuitry instead of the analog circuit components since digital circuitry takes up less physical space and draws less power. A conventional digital PWM control circuit includes a subtractor that produces an error signal representing the difference between a signal to be controlled (e.g., output voltage (Vo)) and a reference voltage. An analog-to-digital converter (ADC) converts the error signal into a digital signal. The digital error signal is provided to a loop compensation filter having a transfer function H(z) that provides stability for the voltage regulator feedback loop. A digital pulse width modulator (DPWM) then produces a proportional pulse width modulated signal that is used to control the power switches of the voltage regulator.
In order to keep the complexity of the PWM control circuit low, it is desirable to hold the number of bits of the digital signal to a small number. At the same time, however, the number of bits of the digital signal needs to be sufficiently high to provide enough resolution to secure precise control of the output value. If the output voltage needs to be programmable through a large range, it is even more difficult to maintain a small DC error on the subtractor and therefore set point accuracy errors will increase. While the circuit can be made accurate over a wide range by providing adjustable gain and offset, this comes with additional cost and complexity. Moreover, the ADC needs to be very fast to respond to changing load conditions and enable fast transient response of the feedback loop. Current microprocessors exhibit supply current slew rates of up to 20 A/μs, and future microprocessors are expected to reach slew rates greater than 350 A/μs, thereby demanding extremely fast response by the voltage regulator. Very often, fast response time and DC precision are contradictory requirements. The bit size of the digital signal also affects the complexity of the digital circuitry that implements the transfer function H(z) and hence the associated cost.
Thus, it would be advantageous to provide a system and method for digitally controlling a switched mode voltage regulator that overcomes these and other drawbacks of the prior art. More specifically, it would be advantageous to provide a double-loop output voltage control circuit for controlling a switched mode voltage regulator using digital circuitry having better repeatability and accuracy.